Battery-powered tools, such as cordless electric lawn mowers, cordless electric mitre saws, cordless electric table saws, etc. have become more widely accepted for their environment-protection qualities and portability. However, the capacity of the battery is limited, thus for example when using a cordless mower, the mowing cannot be accomplished with a single battery pack, or even two battery packs if the area of the lawn is large. Additionally, the battery pack tends to heat up after continuous and circular use, especially in a high-temperature environment, thus it may take longer to recharge the battery. To solve the above problems, a user often prepares several battery packs and a corresponding number of chargers in order to charge several battery packs at once. However, using several battery packs is much more expensive for users.